Mesoporous Nanocrystalline TiO2 Research Articles

Electro- and Photoinduced Interfacial Charge Transfers in Nanocrystalline Mesoporous TiO2 and TiO2/Iron Porphyrin Sensitized Films under CO2 Reduction Catalysis.

Electro- and photochemical CO2 reduction (CO2R) is the quintessence of modern-day sustainable research. We report our studies on the electro- and photoinduced interfacial charge transfer occurring in a nanocrystalline mesoporous TiO2 film and two TiO2/iron porphyrin hybrid films (meso-aryl- and β-pyrrole-substituted porphyrins, respectively) under CO2R conditions. We used transient absorption spectroscopy (TAS) to demonstrate that, under 355 nm laser excitation and an applied voltage bias (0 to -0.8 V vs Ag/AgCl), the TiO2 film exhibited a diminution in the transient absorption (at -0.5 V by 35%), as well as a reduction of the lifetime of the photogenerated electrons (at -0.5 V by 50%) when the experiments were conducted under a CO2 atmosphere changing from inert N2. The TiO2/iron porphyrin films showed faster charge recombination kinetics, featuring 100-fold faster transient signal decays than that of the TiO2 film. The electro-, photo-, and photoelectrochemical CO2R performance of the TiO2 and TiO2/iron porphyrin films are evaluated within the bias range of -0.5 to -1.8 V vs Ag/AgCl. The bare TiO2 film produced CO and CH4 as well as H2, depending on the applied voltage bias. In contrast, the TiO2/iron porphyrin films showed the exclusive formation of CO (100% selectivity) under identical conditions. During the CO2R, a gain in the overpotential values is obtained under light irradiation conditions. This finding was indicative of a direct transfer of the photogenerated electrons from the film to absorbed CO2 molecules and an observed decrease in the decay of the TAS signals. In the TiO2/iron porphyrin films, we identified the interfacial charge recombination processes between the oxidized iron porphyrin and the electrons of the TiO2 conduction band. These competitive processes are considered to be responsible for the diminution of direct charge transfer between the film and the adsorbed CO2 molecules, explaining the moderate performances of the hybrid films for the CO2R.

Visible-light driven of heterostructured LaFeO3/TiO2 photocatalysts for degradation of antibiotics: Ciprofloxacin as case study

In this contribution, a favorable strategy to construct highly effective porous LaFeO3/TiO2 photocatalysts has been developed to decontaminate Ciprofloxacin (CIP) in water under visible illumination. Pluronic L-64 surfactant was utilized for synthesizing mesoporous TiO2 nanocrystalline, and then LaFeO3 at diverse contents has been impregnated on the TiO2 surface. The complete CIP degradation over 3% LaFeO3/TiO2 nanocomposite was determined to be around 100% within 90 min. The photocatalytic efficency of LaFeO3/TiO2 nanocomposite was promoted 20 folds larger than the TiO2 NPs. The rate constant of the 3%LaFeO3/TiO2 nanocomposite (1.58 × 10-2 min−1) was found to be about 31.6 folds higher than TiO2 NPs (0.05 × 10-2 min−1). This was imputed to the superb synergistic effect between LaFeO3 and TiO2 and their magnificent photocatalysis stability. The capability of charge carriers mobility in porous LaFeO3/TiO2 photocatalysts was verified by photoelectrochemical and photoluminescence responses. The improved photocatalytic performance of LaFeO3/TiO2 nanocomposite was significantly interpreted as the formation of 2D - mesoporous LaFeO3/TiO2 heterojunction, which boosted the mobility and separation of photoinduced charges. This study provides a practicable avenue to design and construct highly active TiO2-based nanomaterials under visible illumination.

Simple and Fast Microwave-Assisted Synthesis Methods of Nanocrystalline TiO2 and rGO Materials for Low-Cost Metal-Free DSSC Applications.

Nanocrystalline TiO2 and reduced graphene oxide (rGO) materials have been synthesized by a simple and low-cost microwave-assisted hydrothermal method and applied in dye-sensitized solar cells (DSSCs) as photoactive and metal-free counter electrodes, respectively. Different TiO2 nanocrystalline materials have been synthesized via the acid hydrolysis sol–gel method, followed by microwave hydrothermal treatment at 210 °C and 300 psi and at different microwave irradiation times (20, 30, 45, and 60 min) instead of the usual hydrothermal time of 12 h. The properties of the produced mesoporous nanocrystalline TiO2 are investigated in terms of their morphology, crystal structure, optical properties, and surface area behavior using relevant characterization techniques. Maximum specific surface area values (SBET) of 97.77 and 100.7 m2 g–1 are measured for TiO2, with the average crystallite sizes of 18.6 and 17.5 nm, at microwave irradiation times of 30 and 45 min, respectively. Different rGO samples have been prepared by the modified Hummers method, followed by microwave-assisted reduction at a temperature of 200 °C and pressure of 300 psi at different microwave irradiation times (3, 17, and 25 min). The physicochemical properties of the different rGO samples in terms of morphology, crystallization, and optical properties are characterized by TEM, XRD, and Raman spectroscopic analysis. The current density Jsc of the fabricated DSSCs based on TiO2 as the photoelectrode and rGO as the counter electrode compared with DSSCs based on Pt as the counter electrode is found to be 11.25 and 9.28 mA cm–2, respectively. Although the overall power efficiency of the fabricated DSSCs based on rGO as the counter electrode is lower than that based on the Pt electrode, the former still exhibits promising prospects for replacing Pt with low-cost metal-free carbon-based DSSCs.

Comparative photovoltaic and impedance spectroscopic study on carbon counter electrode based CdS quantum dot sensitized solar cell using polysulfide and iodide/triiodide as redox liquid electrolytes

In this investigation we report comparative study on the quantum dot sensitized solar sells (QDSSCs) using polysulfide and iodide/triiodide as redox liquid electrolytes aimed for the improvement in performance of CdS sensitized solar cell. The CdS films were deposited upon bare FTO and nanocrystalline mesoporous TiO2 coated FTO substrates by following a successive ionic layer adsorption and reaction (SILAR) method. The films were characterized by using atomic force microscopy (AFM) measurements and UV–Visible (UV–vis) spectroscopy. Analysing the AFM images height of the nanocrystals was found to be less than 2.84 nm. The estimated size of the CdS quantum dots (QDs) obtained using the UV–vis data and hyperbolic band model (HBM) was 4.94 nm. Photovoltaic performance and impedance spectroscopic measurements were performed on these FTO/TiO2/CdS/Electrolyte/Carbon type QDSSCs under an AM1.5 illumination (100 mW.cm−2) and dark, respectively. The overall power conversion efficiency, short circuit current and the fill factor of polysulfide based device were observed higher than the iodide/triiodide based cell by 280%, 123%, and 95%, respectively. Improvement in series resistance (Rs) and shunt resistance (Rsh) values of polysulfide based device were observed by 830%, and 348%, respectively. The results achieved using low cost ploysulfide electrolyte and carbon as counter electrode are observed higher than the results of the recent attempts made by other research groups for enhancing performance of the CdS QDSSCs using platinum as counter electrode. The better performance of FTO/TiO2/CdS/Polysulfide/Carbon type QDSSC could be attributed to the higher hole-recovery rate of polysulfide leading to a lower probability for charge recombination and the higher electrocatalytic activity at the CE/electrolyte interface resulting the fast electron injection and transfer from CdS QDs into the photoanode substrate.

Efficiency Enhancement by Insertion of ZnO Recombination Barrier Layer in CdS Quantum Dot-Sensitized Solar Cells.

In this investigation we report the formation of thin ZnO recombination barrier layer at TiO₂/CdS interface aimed for the improvement in performance of CdS sensitized solar cell. The film was deposited upon nanocrystalline mesoporous TiO₂ surface by following a simple chemical process and characterized, using UV-Visible spectroscopy, X-ray diffraction and electron dispersive X-ray measurements. The insertion of ZnO thin layer enhances the QDSC (Quantum dot sensitized solar cell) performance, contributed mainly by an increase in open circuit voltage (Voc) due to reduced electron back transfer from TiO₂ conduction band. Moreover, the analysis of photovoltaic characteristics upon increasing the thickness of the ZnO film reveals that the ZnO recombination barrier layer with optimum thickness at porous TiO₂/CdS interface proved to be an effective potential barrier for minimizing electron back recombination.

Synthesis and characterization of different binary and ternary phase mixtures of mesoporous nanocrystalline titanium dioxide

Different phase compositions of mesoporous nanocrystalline TiO2 (meso-nc-TiO2), comprised of anatase (16–100%), rutile (0–70%) and brookite (0–52%) were obtained by sol–gel synthesis with or without hydrothermal treatment (HTT) by means of titanium tetrabutoxide and dibenzo-18-croun-6 as structure-forming agent in the presence of HCl. It was shown, that small amounts of surfactant and/or lanthanum salt as well as HTT determine phase composition and texture of meso-nc-TiO2. All samples were calcined at 500 оС and characterized by SEM, TEM, XRD and N2-adsorption/desorption isotherms. It has been established that photocatalytic properties of almost all obtained samples significantly exceed the photocatalytic activity of Evonik P-25 TiO2 in gas phase ethanol oxidation. The most active sample is characterized by phase composition of anatase (97%)-rutile (3%). It is obvious, that decrease of photocatalytic activity of sample was affected by decrease of anatase phase content. It was shown that the specific surface area of the sample is not a key factor affecting the activity of mixed-phase meso-nc-TiO2 samples in the process of ethanol oxidation.

Optical Hot Spot Generation by the Plasmonic Coupling of Au Nanoparticles in the Nanospaces of Mesoporous Titanium(IV) Oxide.

An in situ reduction technique consisting of chemisorption of 1,3,5,7-tetramethylcyclotetrasiloxane (TMCTS) and subsequent reaction with HAuCl4 has been developed for depositing Au nanoparticles (NPs) uniformly in the depth direction of a mesoporous TiO2 nanocrystalline film (Au/TMCTS/mp-TiO2). The TMCTS monolayer is further converted into silicon oxide by heating in the air (Au/SiOx/mp-TiO2). In the absorption spectra of Au/SiOx/mp-TiO2 prepared at varying HAuCl4 concentrations (C), the localized surface plasmon resonance (LSPR) band of Au NPs significantly broadens C ≈ 1.22 mM at 546 nm to be split into two peaks around 500 and 700 nm at C ≥ 2.43 mM, whereas such a phenomenon is not observed for the usual Au NP-loaded TiO2 particles. Three-dimensional-finite difference time domain simulations showed that the unique optical property of Au/SiOx/mp-TiO2 stems from the effective LSPR coupling of very close Au NPs and partial fusions in the nanospaces of mp-TiO2. Further, the optical hot spots in Au/TMCTS/mp-TiO2 as well as Au/SiOx/mp-TiO2 generate an intense local electric field giving increase to a great enhancement of the absorption in the infrared spectrum of the TMCTS monolayer on mp-TiO2.

Ultrathin Silicon Oxide Film‐Induced Enhancement of Charge Separation and Transport of Nanostructured Titanium(IV) Oxide Photoelectrode

The development of nanostructured semiconductor electrodes represented by a mesoporous TiO2 nanocrystalline (mp-TiO2 ) film is currently bringing great progresses in photoelectrochemical (PEC) devices for solar-to-electricity and solar-to-chemical conversion. Two serious losses can occur in PEC devices: 1) recombination between the conduction band (CB) electrons and valence band (VB) holes in the bulk and at the surface and 2) back reaction or electron trapping by oxidant in the electrolyte solution during transport to the electron-collecting electrode. Thus, the major challenge in common with the nanostructured semiconductor photoanodes is to achieve efficient charge separation and electron transport. In this study, an ultrathin SiOx layer was formed on both the external and the internal surface of mp-TiO2 using an original chemisorption-calcination technique employing 1,3,5,7-tetramethyltetrasiloxane as a starting material. The SiOx surface modification of the mp-TiO2 photoanode drastically prolongs the mean lifetime of CB-electrons in TiO2 because of enhanced charge separation and electron transport by the negative charge applied in aqueous electrolyte solution. We have demonstrated that the performance of a one-compartment H2 O2 -photofuel cell using mp-TiO2 as the photoanode is greatly boosted by the surface modification with the SiOx layer. We anticipate that this methodology is widely applicable to nanostructured metal oxide semiconductor electrodes, contributing to the improvement in the performance of PEC devices.

Synthesis of novel colorants for DSSC to study effect of alkyl chain length alteration of auxiliary donor on light to current conversion efficiency

Five (MA1-MA5) hemicyanine based sensitizer having N, N-diethyl aniline as a primary donor and hydroxy or alkoxy as an auxiliary donor have been synthesized to establish a correlation between amphiphilic nature of the sensitizer and charge recombination. A strong electron withdrawing 3-(carboxymethyl)-2-methylbenzo[d]thiazol-3-ium bromide has been explored as an acceptor. All the dyes were characterized by 1H-NMR, 13C-NMR and CHN analysis. The photophysical properties of these dyes were recorded in seven different solvents which do not show any significant impact on absorption and emission maxima while molar absorptivity coefficient decreases with increase in alkyl chain length. These dyes show very poor emission in all the solvents. Nano-crystalline mesoporous TiO2 based dye-sensitized solar cells were fabricated using MA1 to MA5 sensitizers to evaluate their photovoltaic performance. MA5 having six carbon alkyl chain shows maximum efficiency of 4.97% while MA1 without any alkyl chain shows the lowest efficiency of 3.40%. As the length of alkyl chain increase efficiency increase due to increment in short circuit current (Jsc) and retardation in the recombination process. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) were explored to obtain vertical excitation, HOMO-LUMO energy and electron density distribution.

An Ultrasonication Based Facile Protocol to Synthesize Mesoporous Nanocrystalline TiO2 as Photo Anode for Application in Quantum Dot/Perovskite Sensitized Solar Cell

Quality of the photo-electrode films employed in the fabrication of Quantum Dot Sensitized Solar Cells (QDSSC) or Perovskite solar cells depends on the quality of dispersion used in preparing these films. The dispersion should be stable and free from aggregations in order to make transparent electrodes. Herein, we discuss one of the most facile and simple methods for fabrication of mesoporous transparent titania as a photo-electrode by using different organic additives through a step wise ultrasonication. The changes that took place in the dispersion were analyzed through each step of the protocol with the help of zeta potential along with discussing the role of each additive used in the protocol. The effective size of the particle in dispersion was determined. The prepared paste was further characterized using Thermo Gravimetric Analysis to understand thermal stability of its constituents. Structural analysis of photo-electrode film was done using Transmission Electron Microscopy, Field Emission Scanning Electron Microscopy and x-ray Diffractometry. Optical absorption studies were employed to determine the band gap of the material. Photocurrent density versus photovoltage curves were also studied in order to evaluate the photovoltaic performance of the fabricated photo-electrode after assembling a QDSSC using the prepared photo-electrode.

In Situ Shape Change of Au Nanoparticles on TiO2 by CdS Photodeposition: Its Near-Field Enhancement Effect on Photoinduced Electron Injection from CdS to TiO2.

Hemisphere-like gold nanoparticles (NPs) were loaded on TiO2 (Au/TiO2) by the deposition–precipitation method. Subsequent photodeposition of CdS on the Au surface of Au/TiO2 at 50 °C yields Au(core)–CdS(shell) hybrid quantum dots with a heteroepitaxial (HEPI) junction on TiO2 (Au@#CdS/TiO2), whereas nonHEPI Au@CdS/TiO2 was formed by CdS photodeposition at 25 °C. In the HEPI system, the shape of the Au core changes to an angular shape, whereas it remains in a hemisphere-like shape in the nonHEPI system. The hot photodeposition technique was applied to the Au NP-loaded mesoporous TiO2 nanocrystalline film (Au/mp-TiO2). Using Au@CdS/mp-TiO2 and Au@#CdS/mp-TiO2 as the photoanodes, two-electrode quantum dot-sensitized photoelectrochemical cells were fabricated for hydrogen (H2) generation from water, and the performances of the cells were evaluated under illumination of simulated sunlight. In the photocurrent and the rate of H2 evolution, the Au@#CdS/mp-TiO2 photoanode cell surpasses the CdS/mp-TiO2 and Au@CdS/mp-TiO2 ones. Three-dimensional finite-difference time-domain calculations for the model systems indicated that the angular shape Au core generates an intense electric field at the corners and edges, extending the electric field distribution over the Au core–CdS shell interface. The striking shape effect on the cell performances can originate from the promotion of the CdS excitation and charge separation due to the near-field enhancement by the deformed Au core.

Photo-effect on the electromotive force in two-compartment hydrogen peroxide-photofuel cell

Mesoporous TiO2 nanocrystalline film was coated on fluorine-doped tin oxide electrode (mp-TiO2/FTO) by the doctor blade method, and Prussian blue film was electrodeposited on FTO (PB/FTO). A two-compartment H2O2-PFC with the structure of mp-TiO2/FTO (photoanode)|0.1 M KClO4 (pH 3)|Nafion (separator)|0.1 M KClO4 with 0.1 M H2O2 (pH 3)|PB/FTO (cathode) was constructed. Under UV-light irradiation, the two-compartment PFC provides short-circuit current (Jsc) = 1.14 mA cm−2 and open-circuit potential (Voc) = 952 mV much greater than the values of Jsc = 0.197 mA cm−2 and Voc = 607 mV for the one-compartment cell. Importantly, the electromotive force for the two-compartment H2O2-PFC exceeds the thermodynamic value of 534 mV calculated from the electrode potentials for the half-reactions.

Dye Excited States Oriented Relative to TiO2 Surface Electric Fields

Two ruthenium compounds were anchored to mesoporous nanocrystalline TiO2 thin films to probe local electric fields generated by TiO2 reduction or cation adsorption in acetonitrile electrolytes. The metal-to-ligand charge-transfer (MLCT) excited states were well formulated as [RuIII(dtb)2(dcb–)]2+* and [RuIII(btfmb)(dcb)(btfmb–)]2+*, where dtb is 4,4′-(tert-butyl)2-2,2-bipyridine, dcb is 4,4′-(CO2H)2-2,2-bipyridine, and btfmb is 4,4′-(CF3)2-2,2-bipyridine. The MLCT excited state was orientated toward the TiO2 surface linker (antiparallel to the field) for Ru(dtb)2(dcb–)*/TiO2 and away from the surface for Ru(btfmb)(dcb)(btfmb–)*/TiO2. Reduction of the TiO2 in an electrochemical cell resulted in a blue shift of the Ru(dtb)2(dcb–)*/TiO2 photoluminescence spectrum, while the Ru(btfmb)(dcb)(btfmb–)*/TiO2 spectrum was either unchanged or was red-shifted. Accompanying these spectral shifts were bimodal changes in the PL intensity and excited-state lifetime that first increased and then decreased as the quasi-Fer...

One-Compartment Hydrogen Peroxide-Photofuel Cell Using TiO2 Photoanode and Prussian Blue Cathode

We have studied the factors affecting the performances of one-compartment hydrogen peroxide photofuel cell (H2O2-PFC) consisting of TiO2 photoanode, cathode, and an aqueous electrolyte solution. As the photoanode, a dense TiO2 film was formed on fluorine-doped tin oxide by a dip-coating method (d-TiO2/FTO) with the thickness controlled below ∼0.5 μm, and a mesoporous TiO2 nanocrystalline film with a thickness of 3.6 μm was coated on FTO (mp-TiO2/FTO) by a doctor blade method. The d-TiO2/FTO provides performances superior to mp-TiO2/FTO in spite that the decrease in the surface area. Building a compact TiO2 underlayer by a spray method (d-TiO2/s-TiO2/FTO) results in further improvement of the cell performances. On the other hand, the replacement of glassy carbon cathode by Prussian Blue-electrodeposited FTO drastically increases the cell performances due to its excellent electrocatalytic activity for the H2O2 reduction. A one-compartment H2O2-PFC with the structure of d-TiO2/s-TiO2/FTO (photoanode) | 0.1 M NaClO4 and 0.1 M H2O2 (pH 3, electrolyte solution) | PB/FTO (cathode) exhibits short-circuit current (Jsc) of 1.57 mA cm−2 and open-circuit voltage (Voc) of 787 mV.

A Unique Disintegration–Reassembly Route to Mesoporous Titania Nanocrystalline Hollow Spheres with Enhanced Photocatalytic Activity

AbstractA novel disintegration–reassembly route is reported for the synthesis of mesoporous TiO2 nanocrystalline hollow spheres with controlled crystallinity and enhanced photocatalytic activity. In this unique synthesis strategy, it is demonstrated that sol–gel‐derived mesoporous TiO2 colloidal spheres can be disintegrated into discrete small nanoparticles that are uniformly embedded in the polymer (polystyrene, PS) matrix by surface‐induced photocatalytic polymerization. Subsequent reassembly of these TiO2 nanoparticles can be induced by an annealing process after further coating of a resorcinol–formaldehyde (RF) resin, which forms self‐supported hollow spheres of TiO2 at the PS/RF interface. The abundant phenolic groups on the RF resin serve as anchoring sites for the TiO2 nanoparticles, thus enable the reassembly of the TiO2 nanoparticles and prevent their sintering during the thermal crystallization process. This unique disintegration–reassembly process leads to the formation of self‐supported TiO2 hollow spheres with relatively large surface area, high crystallinity, and superior photocatalytic activity in dye degradation under UV light irradiation.

Functional Titanium Dioxide-Derived Materials of Different Morphology and Metal–Organic Framework Compounds

New approaches are examined for the creation of functional materials derived from composites of mesoporous nanocrystalline TiO2 with nanoparticles of nickel, copper, and silver, dyes, and also nanocomposites of new metal–organic frameworks formed by azamacrocyclic nickel and zinc complexes and aromatic carboxylates with acridine orange dye and iodine. Spectral, luminescent and electrophysical characteristics of these materials were determined. Examples are given for their possible use as photocatalysts, sensitive elements of luminescent sensors, and proton-conducting membranes.

Tuning of selective solar photocatalysis by Mn2+ decorated nanocrystalline mesoporous TiO2

We report a novel design for visible light activated mesoporous titania (TiO2) for selective photocatalysis. The resulted TiO2 nanomaterials were characterized by Transmission Electron Microscopy (TEM), Powder X-ray Diffraction (PXRD), Fourier Transformed Infra-Red (FTIR) spectroscopy, BET surface area measurement and Ultra violet-visible (UV–Vis) spectroscopy. The selective photocatalysis was tested by measuring the rate of degradation of aqueous dye mixture of methyl orange (MO) and methylene blue (MB), and the rate of selective degradation of the dye mixture was tuned by varying the surface charge on the TiO2. The surface charge was probed with zeta (ζ) potential measurements. The ζ-potential measurement showed that the selectivity of dye degradation depended on the surface charge present on the TiO2 nanomaterial.

Rapid fabrication of mesoporous TiO2 thin films by pulsed fibre laser for dye sensitized solar cells

In this paper we demonstrate for the first time that a fibre laser with a wavelength of 1070 nm and a pulse width of milliseconds can be applied to generate mesoporous nanocrystalline (nc) TiO2 thin films on ITO coated glass in ambient atmosphere, by complete vaporisation of organic binder and inter-connection of TiO2 nanoparticles, without thermally damaging the ITO layer and the glass substrate. The fabrication of the mesoporous TiO2 thin films was achieved by stationary laser beam irradiation of 1 min. The dye sensitized solar cell (DSSC) with the laser-sintered TiO2 photoanode reached higher power conversion efficiency (PCE) of 3.20% for the TiO2 film thickness of 6 μm compared with 2.99% for the furnace-sintered. Electrochemical impedance spectroscopy studies revealed that the laser sintering under the optimised condition effectively decreased charge transfer resistance and increased electron lifetime of the TiO2 thin films. The use of the fibre laser with over 40% wall-plug efficiency offers an economically-feasible, industrial viable solution to the major challenge of rapid fabrication of large scale, mass production of mesoporous metal oxide thin film based solar energy systems, potentially for perovskite and monolithic tandem solar cells, in the future.

Plasmonic effect in Au(core)-CdS(shell) quantum dot-sensitized photoelectrochemical cell for hydrogen generation from water

UV-light irradiation of Au nanoparticle (NP)-loaded mesoporous TiO2 nanocrystalline films in an ethanol solution of Cd(NO3)2 and S8 at 298 K gives rise to selective CdS deposition on Au NPs to yield Au(core)-CdS(shell) hybrid quantum dots (Au@CdS/mp-TiO2). Two-electrode quantum dot-sensitized photoelectrochemical cells with the structure of photoanode|0.25 M Na2S, 0.35 M Na2SO3 (solvent = water)|cathode were fabricated. The Au@CdS/mp-TiO2 photoanode cell yields hydrogen (H2) with a rate of 0.18 ml h−1 [solar-to-current efficiency (STCE) = 0.028%] without external bias far surpassing the rate of 0.028 ml h−1 (solar-to-current efficiency = 0.006%) for the CdS/mp-TiO2 photoanode cell under illumination of simulated sunlight (λ > 430 nm, AM 1.5, one sun). The 3D finite-difference time-domain calculations for a model Au@CdS/TiO2 system indicate that an intense local electric field with an enhancement factor of ∼103 is generated at the Au-CdS-TiO2 three-phase interface. The striking effect of the Au core stems from the enhancement of the excitation of the CdS shell and subsequent charge separation by the intense local electric field.

Photocatalytic Synthesis of CdS(core)-CdSe(shell) Quantum Dots with a Heteroepitaxial Junction on TiO2 : Photoelectrochemical Hydrogen Generation from Water.

A major challenge in chemistry for the synthesis of hetero-nanostructures is to build up atomically commensurate interfaces for smooth interfacial charge transfer. Photodeposition of CdSe on a CdS-preloaded mesoporous TiO2 nanocrystalline film yields CdS(core)-CdSe(shell) quantum dots (CdS@CdSe/mp-TiO2 ) with a heteroepitaxial nanojunction at 298 K. Two-electrode quantum-dot-sensitized photoelectrochemical (QD-SPEC) cells with the structure photoanode |0.25 M Na2 S, 0.35 M Na2 SO3 (solvent=water)| cathode were fabricated. The CdS@CdSe QD-SPEC cell affords a solar-to-current efficiency (STCE) of 0.03 % without external bias under illumination of simulated sunlight (λ>430 nm, AM 1.5, one sun). By applying 0.1 V between the electrodes, the STCE increases up to 0.048 %, far surpassing the CdS/mp-TiO2 and CdSe/mp-TiO2 photoanode cells. The CdS-CdSe interfacial analysis by high-resolution transmission electron microscopy and the band energy analysis taking the size quantization and the electrolyte effect indicate that the excellent performance of the CdS@CdSe/mp-TiO2 photoanode originates from the effective charge separation due to the cascade-like band edge alignment and the heteroepitaxial junction between CdS and CdSe QDs. In addition, high surface coverage of TiO2 with QDs can contribute to the reduction in the loss of the electron transport from TiO2 to the electron collecting electrode.